Stannous ions and opper ions are known to inhibit dental plaque formation and reduce the acidogenicity of dental plaque in humans and in animals. In the present study copper ions reduced caries in rats when applied topically or added to the drinking water; this appeared to be related to the decreased numbers of S. mutans in the animals. Stannous ions, supplied as stannous fluoride, also reduced both caries and numbers of S. mutans. Stannous tartrate and stannous chloride did not reduce dental caries in rats; these results may be due to the low concentration of stannous ions available at the acidic pH values of the test solutions.